Functional fiber: Fiber that is extracted and isolated from whole foods, then added to processed foods.

However, there is a major problem with classifying fiber in this way. It tells us absolutely nothing about their health effects.

A popular alternative method is to classify fiber based on its solubility (soluble vs insoluble), viscosity (viscous vs non-viscous) and fermentability (fermentable vs non-fermentable).

There is another class of nutrients called resistant starches, which are classified as dietary fibers.

Fibers are indigestible carbohydrates found naturally in plant foods. They are often classified as either dietary (found naturally) or functional (added to foods).

Soluble vs Insoluble Fiber

The solubility of fiber refers to its ability to dissolve in water.

Soluble fiber blends with water in the gut, forming a gel-like substance. It can reduce blood sugar spikes, and has various metabolic health benefits (4).

Insoluble fiber does not blend with the water and passes through the digestive system mostly intact. It functions mostly as a “bulking” agent and may help speed the passage of food and waste through your gut (5).

The best whole-food sources of fermentable fibers are beans and legumes. A 1-cup serving often provides up to half of the recommended daily intake of fiber.
The fact is, the by-product of fiber fermentation is gas. This is why foods high in fermentable fiber can cause flatulence and stomach discomfort. Furthermore, fermentable fiber is digested by the friendly bacteria in the gut and used for energy. This can lead to various beneficial effects on health.

Viscous Fiber

Some types of soluble fibers form a thick gel when they blend with water, known as viscous fibers.

Put simply, the viscosity of a fluid refers to its “thickness.” For example, honey is more viscous than water.

For Example, when you eat viscous fiber, it forms a gel-like substance that “sits” in the gut.

This slows down the digestion and absorption of nutrients, resulting in a prolonged feeling of fullness and reduced appetite (16, 17, 18).

A review of 44 studies on fiber treatments found that only viscous fibers reduced food intake and caused weight loss (19).